The regular Impreza got the 2.2. The RS version got the 2.5. They are both phase 2 motors, both SOHC. The 2.2 does not usually eat head gaskets like the 2.5, but the phase 2 is more prone to it than previous 2.2's. But they are still generally tougher and less problematic than the 2.5's. But, general maintenance comes into play as well!